(Supported in part by NSF DPP89-17375 to S. Bowser) Almost every cell in the human body contains a single primary cilium that extends from the mature centriole of the centrosome. In some tissues, e.g., kidney epithelia, the cilia projects many micrometers from the cellular surface. Because of their diminutive size (0.25lm thick) primary cilia are difficult to study in vivo and their function remains unknown. We have developed the first method for viewing primary cilia in living cells. It is based on growing kidney cells on Formvar films, and then folding these films between two coverslips. This allows individual living cells, and their associated primary cilia, to be examined along with length for considerable periods by video-enhanced DIC LM. Using this approach we are studying the material properties and motility associated with primary cilia. We are also using low-light level fluorescence LM, in conjunction with DIC, to examine surface chemistry and cytoplasmic exc hange within primary cilia. Same-cell correlative LM/HVEM is used to examine ultrastructural correlates of the LM findings.